1. Field of the Invention
The present invention relates generally to sense amplifier circuitry.
2. Description of the Related Art
Sense amplifiers are often used to detect and amplify a data signal present on a differential pair of data lines. For example, a sense amplifier can be used to detect a higher signal present on a differential pair of data lines, amplify the higher signal, and output the amplified signal for processing by other circuitry. Sense amplifiers are commonly used in this manner to discriminate between signals present on complementary bitlines of memory cell arrays.
FIG. 1 is an illustration showing a conventional current mode sense amplifier, in accordance with the prior art. The conventional sense amplifier includes input nodes 101 and 103 that are connected to receive a differential pair of input signals. Input nodes 101 and 103 are also connected to an output of PMOS devices 105 and 107, respectively. The inputs of PMOS device 105 and 107 are connected to a supply voltage. The gates of PMOS devices 105 and 107 are connected to ground. Thus, PMOS devices 105 and 107 are always transmitting from the supply voltage to the input nodes 101 and 103, respectively.
The conventional sense amplifier also includes two PMOS devices 109 and 111 that have their inputs connected to input nodes 101 and 103, respectively. The outputs of PMOS devices 109 and 111 are connected to sense nodes 113 and 115, respectively. Also, a gate of PMOS device 109 is connected to the output of PMOS device 111. Conversely, a gate of PMOS device 111 is connected to the output of PMOS device 109. In this manner, PMOS devices 109 and 111 are cross-coupled. Thus, PMOS devices 109 and 111 serve to provide positive feedback to each other.
The conventional sense amplifier further includes two NMOS devices 117 and 119 that have their inputs connected to sense nodes 113 and 115, respectively. The outputs of NMOS devices 117 and 119 are connected to ground. Also, a gate of NMOS device 117 is connected to an input of NMOS device 119. Conversely, a gate of NMOS device 119 is connected to an input of NMOS device 117. Hence, NMOS devices 117 and 119 are cross-coupled. Therefore, NMOS devices 109 and 111 serve to provide positive feedback to each other.
The conventional sense amplifier further includes an NMOS device 121 having a first terminal connected to the sense node 113 and a second terminal connected to the sense node 115. Prior to performing a sensing operation, the NMOS device 121 is controlled to transmit such that sense nodes 113 and 115 are afforded an opportunity to equalize. During the sensing operation, the NMOS device 121 is controlled to isolate sense nodes 113 and 115 from each other. Thus, prior to performing the sensing operation, sense nodes 113 and 115 are intended to maintain an equal voltage. Then, during the sensing operation, input nodes 101 and 103 will be influenced by the incoming differential pair of input signals such that either input node 101 or 103 will begin to attain a lower state. If the input node 101 begins to attain a lower state relative to the input node 103, the output of PMOS device 111 will become greater, thus causing transmission of PMOS device 109 to decrease. In following, a decrease in the transmission of PMOS device 109 causes transmission of the PMOS device 111 to increase further. If the input node 103 begins to attain a lower state relative to the input node 101, the PMOS devices 109 and 11 will operate in a complementary manner relative to that previously described with respect to the input node 101 attaining the lower state. Also, during the sensing operation, as either sense node 113 or 115 begins to attain a higher state, transmission through either NMOS device 119 or 117, respectively, will begin to increase. Thus, as either sense node 113 or 115 begins to attain a higher state, the other sense node will be pulled to ground. Therefore, a higher signal on input node 101 relative to input node 103 will cause sense node 113 to attain a high state and sense node 115 to attain a low state, vice-versa.
The conventional sense amplifier as described with respect to FIG. 1, however, is vulnerable to a number of adverse effects and limitations. One such adverse effect is associated with Negative Bias Temperature Instability (NBTI). NBTI is a deep submicron technology phenomena that causes a threshold voltage of a PMOS device to increase over time. NBTI effects are influenced by gate-to-source bias conditions and temperature during the silicon lifetime of the PMOS device. With respect to the conventional sense amplifier, the increase in threshold voltage of PMOS devices resulting from NBTI impacts at least two aspects of circuit performance. In one aspect, the increase in threshold voltage reduces the driving capability of the PMOS devices, thus delaying the conventional sense amplifier circuit operation. In another aspect, since NBTI effects are influenced by gate-to-source bias conditions, one of the cross-coupled PMOS devices 109 and 111, with respect to FIG. 1, will necessarily experience different NBTI effects relative to the other. Thus, the increase in threshold voltage caused by NBTI will be uneven between PMOS devices 109 and 111. A threshold voltage imbalance between PMOS devices 109 and 111 requires longer signal development time to overcome the imbalance, thus further delaying the conventional sense amplifier circuit operation. Also, as technology evolves to more aggressive supply voltage levels, NBTI induced threshold voltage shifts will become even more significant, leading to intolerable conventional sense amplifier performance penalties.
Another limitation of the convention sense amplifier of FIG. 1 is associated with low voltage operability. The lowest voltage at which the conventional sense amplifier can operate is determined by the threshold voltage of NMOS device 121. The gate bias of NMOS device 121 needs to be greater than two times its the threshold voltage to achieve equilibrium between sense nodes 113 and 115. Typical threshold voltages for NMOS devices are within a range extending from about 300 mV to about 350 mV. Thus, NMOS device 121 requires a substantial gate bias to equalize sense nodes 113 and 115. Also, the sense nodes 113 and 115 must be equalized well for the conventional sense amplifier to operate at low voltages.
In view of the foregoing, there is a need for a current mode sense amplifier that is tolerant to NBTI effects and is capable of operating at lower voltage levels.